In a conventional tubular member of the above-described type, an intermediate layer thereof is formed by winding prepreg of PAN type carbon fiber. PAN type carbon fibers are manufactured from acrylic fibers, and are widely used in sporting goods, aerospace, industrial and other applications. PAN type carbon fibers display a good balance of high specific strength and modulus of elasticity.
With the above construction, the tubular member provides a progressively higher modulus of elasticity with increase in a bending amount of the member, thus tending to lose its flexibility gradually. For instance, Table 1 and FIG. 1 show ratios a between measured values of flexion amount and theoretically calculated values (i.e. theoretical values) of the same. For obtaining the theoretical flexion amount values, a modulus of elasticity obtained by a flexion experiment using a flat plate is newly calculated through its conversion into a volumetric content of the amount of carbon fiber, and then from this converted bending elastic modulus (shown, e.g., in Tables 1 and 2 below) the theoretical value is derived.
TABLE 1 ______________________________________ Bending .alpha. = measured Elastic Load Flexion W (mm) calculated .times. Modulus (g) measured calculated 100 (%) (ton/mm2) ______________________________________ PAN 200 80.3 84.0 95.5 60 ton/mm.sup.2 * 400 166.8 177.6 93.9 29.90 600 259.3 276.4 93.8 PAN 200 87.0 86.9 100.1 55 ton/mm.sup.2 400 181.3 183.8 98.6 28.50 600 281.5 288.6 97.5 PAN 200 91.0 94.0 96.8 50 ton/mm.sup.2 400 191.3 198.8 96.2 25.00 600 298.0 312.1 95.5 PAN 200 97.5 96.8 100.7 46 ton/mm.sup.2 400 203.8 204.9 99.4 23.40 600 317.3 321.7 98.6 PAN 200 106.3 107.9 98.5 40 ton/mm.sup.2 400 221.0 228.5 98.7 19.50 600 344.0 358.9 95.8 PAN 200 110.3 114.7 96.1 35 ton/mm.sup.2 400 230.3 245.1 93.9 17.60 600 359.5 384.9 93.4 PAN 200 124.8 125.0 99.8 30 ton/mm.sup.2 400 263.5 269.6 97.7 15.30 600 410.0 419.9 97.6 ______________________________________ *Figures indicate fiber modulus of elasticity.
However, in many cases, the measured flexion amount value and the theoretical flexion amount value do not agree with each other due to, e.g., possible variations in the amount of carbon fiber or the amount of resin due to the composition of the employed materials. As may be clearly understood from FIG. 1, in the case of a tubular member comprised of PAN carbon fiber prepreg, the ratio .alpha. between the measured flexion amount value and the theoretical flexion amount value becomes smaller with increase of the load used in the experiment, i.e. with increase in the amount of flexion. This means that with an increase in the flexion amount the measured elastic modulus becomes higher than that used in the theoretical calculation, thus resulting in decrease of flexibility.
For this reason, with the conventional fishing rod having an intermediate layer formed of PAN carbon fiber, when the rod is bent by pulling a fish, the rod loses its flexibility. The rod thus fails to provide sufficient elasticity to cope with a relatively strong pulling force from the fish, whereby breakage of the fishing line or detachment of the hook from the fish tends to occur. Further, because of the difference between the flexion amount when a fish is hooked and that when no fish is hooked, the rod provides different feels, which make the fishing activity per se difficult.
Similarly, if the above-described tubular member is employed as a shaft of a golf club, when the face of the club is hard struck deep into the ground, the club shaft is significantly bent and loses much of its flexibility at the same time. Hence, the club shaft may be readily damaged thereby.
One conceivable method to solve the above-described inconvenience is use of the pitch carbon fiber along the axial (or longitudinal) direction of the intermediate layer. Pitch carbon fibers, used in spacecraft and other applications, are produced from highly refined petroleum or coal tar pitch, and are characterized by high thermal conductivity and low coefficient of thermal expansion, among other properties. The modulus of elasticity of pitch carbon fibers can range from 60 to 900 GPa.
Table 2 and FIG. 2 show characteristics relating to flexion amounts W of rods having an intermediate layer formed of prepreg containing pitch type carbon fiber impregnated with resin.
TABLE 2 ______________________________________ Bending .alpha. = measured Elastic Load Flexion W (mm) calculated .times. Modulus (g) measured calculated 100 (%) (ton/mm2) ______________________________________ pitch 200 89.8 94.7 94.8 60 ton/mm.sup.2 * 400 190.8 200.4 95.2 27.70 600 307.0 314.6 97.6 pitch 200 86.8 87.4 99.3 65 ton/mm.sup.2 400 182.0 184.8 98.5 31.00 600 288.3 290.1 99.4 pitch 200 77.5 83.6 92.7 70 ton/mm.sup.2 400 163.5 176.8 92.5 33.50 600 257.5 275.1 93.6 pitch 200 75.5 79.6 94.8 75 ton/mm.sup.2 400 159.5 166.9 95.6 36.00 600 250.3 261.8 95.6 ______________________________________ *Figures indicate fiber modulus of elasticity.
Referring to FIG. 2, relative to the ratio between the measured flexion amount and the theoretical flexion amount for a load of 200 g, the ratio .alpha. remains substantially unchanged for a load of 400 g. Further, in comparison with the case of 400 g load, the characteristics improve in all the respects for the load of 600 g. This means that the tubular member still retains sufficient elasticity when the flexion amount W, i.e. the magnitude of bending is significantly increased. Hence, if this tubular member is used as a fishing rod, this rod may sufficiently cope with a sudden strong pulling force from fish.
However, if the tubular member is formed solely of the pitch carbon fiber, the member cannot provide sufficient mechanical strength while being capable of providing high elasticity. If this tubular member is subjected to a bending force, a bent portion of the tubular member is squeezed so that the cross section of the tubular member is deformed into a substantially oval shape.
With this, an inner layer of the major-axis portion of the substantially oval cross section is subjected to a compressing force, while an outer layer of the same is subjected to a tensile force. If such bending force is intensified, cracking tends to occur in the tubular member.
Thus, in order to allow the tubular member to be used as, e.g., a fishing rod, the member needs to have a high modulus of elasticity while retaining a necessary amount of mechanical strength.
Table 3 shows characteristics of typical pitch type and PAN type carbon fibers. As may be seen from Table 3, the PAN type carbon fiber provides a higher compression strength than the pitch type carbon fiber, while providing a lower modulus of tensile elasticity than the latter.
TABLE 3 ______________________________________ tensile elastic compression strength modulus of fiber in fiber orientation ______________________________________ pitch carbon fiber 60 ton/mm.sup.2 53 kg/mm.sup.2 PAN carbon fiber 40 ton/mm.sup.2 120 kg/mm.sup.2 ______________________________________
Accordingly, a primary object of the present invention is to overcome the above-described drawbacks of the conventional art by providing a tubular member for use in, e.g., a fishing rod or a golf club, which is superior in both controllability and strength.